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General Discussion >> Hurricane Ask/Tell

CoconutCandy
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Reged: Fri
Posts: 245
Loc: Beautiful Honolulu Hawaii
'Visualizing' a Ridge of High Pressure
      Sun Sep 02 2007 02:45 AM

>> Can someone please tell me where I can see a visual of a ridge, and explain to me what to look for?

(Reply to 'Texas Cane Tracker' from Post 77721. Apologies to the Mets. Any Mets feel free to extrapolate.)

Well, that's just it. You can't really 'see' a ridge. Since a ridge is high pressure, with sinking air and fair weather (as opposed to a trough of low pressure, with its' rising air and easily-visible stormy weather), there really isn't much to see, as air is normally transparent.

But what you *can* see is the effects of the ridge on the surrounding stratocumulus cloud fields over huge areas of open ocean.

You see, a ridge is an elongated area of high pressure, often extending for hundreds or even thousands of miles, usually east/west, and is sometimes anchored to a high pressure cell at one or both ends, but not always.

And since air flows *outward* from high pressure (inward towards low pressure) winds below (south of) the ridge "axis" blow generally from the E or ENE or NE, and, on the north side of the ridge axis, they blow from the W or WSW or SW.

It is the ridge, which is a semi-permanent feature in the summer months, that provides us the trade winds over the open oceans. Life without the tradewinds (they occasionally die down for various reasons) can be miserable, as they truly are natures' air-conditioning.

So, if you view a large area of ocean on a satellite map (visible daytime loop is better) which is clear and free of storms, you can see huge areas where the cumulus clouds are flowing along from the E or NE. But north of that area, you may see the clouds blowing in the opposite direction, from the W or SW. The broad, elongated, sometimes cloud free area which separates the two, is the ridge axis itself, which is sinking, dry air thus doesn't really provide anything to 'see'.

Interestingly, the winds right along the ridge axis itself, since the air there is sinking and warming, are practically non-existent, with just gentle zephyrs at best. Centuries ago, in the old days of the sailing ships, the ships would sometimes become 'becalmed' for days or even weeks at a time, and the ships made little or no progress in their journeys. In an attempt to 'lighten the load' they would literally throw horses, destined for the new world, overboard (!) so that what little wind there was would be better able to get the ships moving again. The latitude along 30N, where the subtropical ridge usually resides, actually became known, for a time, as the "Horse Latitudes". I wonder just how many horses met their un-timely demise in this fashion over the centuries. Anyone care to chime in with more details on this topic?

But, just because you can't see it, (except indirectly, by it's 'reflection' on the motions of the cloud fields over huge areas of ocean) the ridge is incredibly important because, as mentioned, it provides the trade winds, *AND* it 'steers' storms and hurricanes located below the ridge axis west or west-northwest.

Whenever you see a storm or hurricane moving west or west-northwest, as Hurricane Felix is currently doing, it is the effect of the all-important ridge gently 'pushing' it along and preventing it from moving very much northward.

When the ridge 'runs out' in it's horizontal extent, breaks down, or is eroded away, there forms, instead, a 'weakness' which the storms/hurricanes will tend to move towards. That's when the storms will turn NW or N or even NE, because the ridge is no longer there and no longer 'pushing' them westward.

Here's a good article from Wikipedia that should help you understand and 'visualize' the Subtropical ridge.

http://en.wikipedia.org/wiki/Subtropical_ridge

There's even a nice picture of the East/Central Pacific and the Hawaiian Islands in the article, showing the importance of the subtropical ridge in providing Hawaii with our nearly constant breezy cooling tradewinds.

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